1. Field of the Invention
The present invention relates to a multi-layer polymer electrolyte membrane, an electrochemical apparatus and a process for the preparation of a multi-layer polymer electrolyte membrane.
2. Description of the Related Art
As examples of an electrochemical apparatus including polymer electrolyte membrane, there are solid polymer electrolyte type fuel cell, direct methanol type fuel cell, water and chloro-alkali electrolyzer, ozonizer utilizing these mechanisms, oxygen separator, hydrogen separator, and oxygen sensor.
Such electrochemical apparatuses have a polymer electrolyte membrane which acts as an electrolyte and electrodes which act as anode and cathode, respectively, on the respective surfaces of the polymer electrolyte membrane. The polymer electrolyte membrane and these electrodes may or may not be assembled to each other. In such electrochemical apparatuses, reaction materials react at the cathode and the anode, electrochemically. In this reaction, ions conduct through the polymer electrolyte membrane while electrons conduct through the external circuit.
In the solid polymer electrolyte type fuel cell, for example, oxygen is supplied as a cathode reaction material while hydrogen is supplied as an anode reaction material, so that electric power is given to the external circuit. In the direct methanol type fuel cell, oxygen is supplied as catholyte while a mixture of methanol and water is supplied as anolyte.
Referring to the structure of such an electrochemical apparatus, an electrode having a catalyst layer and a diffusion layer is assembly D on both surfaces of a polymer electrolyte membrane, so that the catalyst layer comes into contact with the polymer electrolyte membrane. The laminate is disposed interposed between a pair of gas-impermeable separators each having a gas flow ditch formed therein to form a single cell as basic unit. A plurality of these single cells are assembly D to form the electrochemical apparatus.
The catalyst layer is comprised of a catalyst such as particulate platinum group metal catalyst and carbon powder having a particulate platinum group metal supported thereon, optionally having a polymer electrolyte added therein. The diffusion layer of the assembly is comprised of an electrically conductive substrate having a high fluid transmission such as hydrophobic carbon paper.
As the foregoing binder there may be normally used a fluororesin such as polytetrafluoroethylene (PTFE). Such a fluororesin acts also as a hydrophobe for rendering the catalyst layer properly hydrophobicity.
In such an electrochemical apparatus having a polymer electrolyte membrane, electrochemical reaction proceeds at the interface between the electrode which acts as a cathode or anode and the polymer electrolyte membrane. Thus, increasing of the interfacial area between the electrode and the polymer electrolyte membrane needs to be as great as possible for the purpose of enhancing the efficiency of the electrochemical apparatus.
To this object such as increasing of the interfacial area, various processes to form the roughening on the surface of a polymer electrolyte membrane have been proposed. For example, JP-A-3-158486 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d) discloses a process involving the use of roughened rolls. JP-A-4-169069 discloses a process involving sputtering. JP-A-4-220957 discloses a process involving plasma etching. JP-A-6-279600 discloses a process including embedding a cloth on a polymer electrolyte membrane, and then peeling the cloth off the polymer electrolyte membrane.
To the same aim, a process involving the formation of the pores in the surface of a polymer electrolyte membrane has been proposed. For example, JP-A-58-7432 discloses a process which has crystallizing the dispersion medium having a polymer electrolyte membrane dissolved therein into droplets which are then removed. JP-A-62-146926 discloses a process which has dispersing particles in a polymer electrolyte membrane, and then removing the particles. JP-A-5-194764 discloses a process which has mixing a polymer electrolyte membrane with a low molecular organic material, and then removing the low molecular organic material.
The polymer electrolyte membrane) having such electrochemical apparatuses is a polymer membrane which exhibits ionic conductivity. The polymer electrolyte membranes, e.g. perfluorosulfonic acid resin membrane, which is an ion exchange resin membrane exhibits proton conductivity in hydrous state. However, it exhibits a decrease of proton conduction with a decrease of water content and exhibits no proton conduction when dried. On the other hand, in the solid polymer electrolyte type fuel cell, etc., proton generated at the anode is conducted with a few hyrorate water molecules through the polymer electrolyte membrane to the cathode, where they react with oxygen to produce water.
Thus, in such an electrochemical apparatus, the water content of the polymer electrolyte membrane at the anode due to permeation of hydorate water and the proton conduction of polymer electrolyte membrane decrease. Accordingly, it is necessary that the water content of the polymer electrolyte membrane be kept high to keep the function of the polymer electrolyte membrane sufficient. To this aim, various proposals have been made.
These proposals are a process which has supplying a humidified fuel gas into the anode in order to supply water into the polymer electrolyte membrane; a process which has reducing the thickness of the polymer electrolyte membrane so that the effect of back diffusion of water caused the gradient of water content between the anode and the cathode is accelerated, whereby the polymer electrolyte membrane is supplied with water; a process which has allowing one end or a part of the polymer electrolyte membrane to come in contact with water to supply water into the polymer electrolyte membrane with water; a process which has allowing fibers or other hygroscopic materials embedded in the polymer electrolyte membrane to come in contact with water at one end thereof, to supply water into the polymer electrolyte membrane with water; a process which has dispersing fine particles of titanium dioxide or the like in the polymer electrolyte membrane to enhance the water retention thereof; and a process which has dispersing fine particles of platinum in the polymer electrolyte membrane so that oxygen and hydrogen permeated through the polymer electrolyte membrane produce water.
The conventional process which has roughening the surface of the polymer electrolyte membrane to increase the contact area is disadvantageous in that the resulting roughness is too large to sufficiently increase the area of contact between the polymer electrolyte membrane and the electrode.
Further, the conventional process which has forming pores in the surface of the polymer electrolyte membrane to increase the contact area is disadvantageous in that the dispersion medium, particles, low molecular organic material, etc. can hardly be completely removed. Thus, the resulting residues prevent the contact of the polymer electrolyte membrane with the electrode or ionic conduction. Further, heating or solvent treatment at the removal step causes the deterioration of the polymer electrolyte membrane that and the reduction of the ionic conduction.
Moreover, the conventional process which has keeping the water content of the polymer electrolyte membrane high has various disadvantages. For example, the process involving the reduction of the thickness of the polymer electrolyte membrane is disadvantageous in that the strength of the polymer electrolyte membrane is lowered. The process which has allowing the polymer electrolyte membrane to come in contact with water and the process involving the utilization of wick are disadvantageous in that a sufficient water retention cannot be obtained. The process involving the dispersion of titanium dioxide or platinum is disadvantageous in that the use of the expensive metal adds to the production cost.
It is an object of the present invention to solve the foregoing conventional problems and hence allow the production of a high-performance electrochemical apparatus.
According to a first aspect of the present invention, a multi-layer polymer electrolyte membrane has a polymer electrolyte layer and a porous polymer electrolyte layer with three-dimensioned network pores on the former polymer electrolyte layer.
According to a second aspect of the present invention, in the multi-layer polymer electrolyte membrane as defined in the first aspect, the polymer electrolyte layers are provided on both side of the porous polymer electrolyte layer with three-dimensional network pores.
According to a third aspect of the present invention, an electrochemical apparatus has a multi-layer polymer electrolyte membrane as defined in the first or second aspect of the present invention provided interposed between electrodes.
According to a fourth aspect of the present invention, a solid polymer electrolyte type fuel cell has a multi-layer polymer electrolyte membrane as defined in the second aspect of the present invention provided interposed between electrodes.
According to a fifth aspect of the present invention, a process for the preparation of a multi-layer polymer electrolyte membrane having a polymer electrolyte layer and a porous polymer electrolyte layer with three-dimensional network pores formed thereon, has the steps of applying a solution of a polymer electrolyte in a solvent containing an alcohol to a polymer electrolyte layer, and then dipping the polymer electrolyte layer to which the solution has been applied in an organic solvent having a polar group other than alcoholic hydroxyl group.